I. Field of the Invention
This invention relates to a process of producing an aluminum sheet article. More particularly, the invention relates to such a process for producing sheet articles made of non heat-treatable alloys suitable for shaping by press forming, particularly 5000 series aluminum alloys suitable for use, for example, in manufacturing automotive panels.
II. Description of the Prior Art
Aluminum alloys of the 5000 series (i.e. those having magnesium alone as the principal alloying element) are commonly used for the fabrication of automotive panels (fenders, door panels, hoods, etc.) and, for such applications, it is desirable to provide alloy sheet product having high yield strength and high ductility. Aluminum alloy sheet articles of suitable gauge and yield strength can be produced by continuous casting followed by rolling to gauge. In a traditional continuous casting process, the metal emerging from the caster is hot and warm rolled to an intermediate gauge and is then coiled (at a temperature of about 300.degree. C.) and transported to another mill (which may be at another plant) and cold rolled to final gauge at a temperature that does not exceed 160.degree. C.
For clarification, it should be mentioned at this point that the term "hot rolling" conventionally means rolling carried out at a temperature above the recrystallization temperature of the alloy, so that the alloy recrystallizes by self-anneal either between roll passes or in the coil after rolling. The term "cold rolling" conventionally means work rolling with substantial work hardening rates such that the alloy exhibits neither recrystallization nor substantial recovery during or after rolling. The term "warm rolling" means rolling carried out between the two, i.e. such that there is no recrystallization but such that the yield strength is reduced substantially due to a recovery process. For aluminum alloys, hot rolling is carried out above 350.degree. C., and cold rolling is carried out below 150.degree. C. Obviously, warm rolling is carried out between 150 and 350.degree. C.
Unfortunately, the conventional process mentioned above is cumbersome and expensive in that intermediate coiling, storage and transportation are required to obtain a sheet article having a suitable microcrystalline structure to produce the desired yield strength.
In U.S. Pat. No. 5,514,228, which issued on May 7, 1996 to Kaiser Aluminum & Chemical Corporation, inventors Wyatt-Mair et al. disclose an in-line continuous casting process in which the sheet is rolled to final gauge without an intermediate coiling step. However, a solution heat treatment step is required ahead of the final rolling pass, such that the sheet is continuously fully annealed prior to final coiling. Unfortunately, 5000 series alloys cannot be strengthened by solution heat treatment in the way contemplated by Wyatt-Mair et al.
In Japanese patent disclosure JP 7-41896, published on Feb. 10, 1995 in the name of Sky Aluminum Co., Ltd., inventors Kamishiro et al. discloses a direct chill (DC) casting process for what may or may not be 5000 series alloys (this is not stated explicitly), in which a warm rolling step is provided between hot rolling and cold rolling steps. The warm rolling step results in partial annealing of the sheet at temperatures in the range of 100 to 350.degree. C. However, the sequence of steps is discontinuous in that the sheet is coiled at least between the hot and cold rolling stages. Also, the aim of the warm rolling step appears to be to improve formability, as opposed to improving yield strength.
There is therefore a need for a process of producing sheet articles of 5000 series aluminum alloys, and other non heattreatable aluminum alloys, on a continuous basis while obtaining alloy sheet products of high yield strength.